This invention relates generally to electrical power converters and more particularly to a current source converter coupled to an AC bus for powering a load.
This invention is directed to an application including a relatively high powered current source known in the art as a Superconducting Magnetic Energy Storage System (SMES) wherein energy is stored in the magnetic field supported by current flowing in a superconducting solenoidal coil. Such a system is known to those skilled in the art and has recently been developed to energize a relatively high power laser system via a polyphase power converter which operates not only to charge the coil from an AC power source such as an electrical power utility grid, but also thereafter to convert current from the SMES coil for either feeding energy back to the utility, or powering the laser through a rectifier located between the AC terminals of the converter and the laser.
In an SMES application, current harmonics are produced by the converter and the rectifier feeding the laser. These harmonics will be small compared to the relatively high powered fundamental current if the current and the load rectifier are of a multi-pulse design. These elements can easily be multi-pulse, 24 pulse or higher, because the large rating of both the converter and rectifier make them economical to be designed as a multi-pulse system. For a twenty-four pulse rectifier or converter, the lowest harmonic in the current is the twenty third and the amplitude of this largest harmonic is typically less than 5%. Because the SMES converter is a current source, however, it cannot absorb the harmonic currents drawn by the load. Thus there must be a capacitor coupled across the utility or AC terminals of the converter to supply the load harmonics and to absorb the converter harmonics.
When the converter is operated disconnected from the utility grid for feeding the laser load, it must operate in a mode in which it controls its own voltage and frequency, independent of the load. Since an SMES converter is an AC current source and not a voltage source, it cannot regulate voltage directly. While it can regulate current into the connected load in a manner which produces a constant voltage, the controls to accomplish this function are relatively difficult to implement if the nature of the load is unknown or time variable. Thus one means for easing the design of the controller for the converter is to place a capacitor across the AC terminals of the converter. The converter controls then regulate the component of current which flows into the capacitor and hence regulate terminal voltage.